wireless: Skip directory when generating certificates

Commit 715a1233 ("wireless: don't write C files on failures") drops
the `test -f $$f` check. The list of targets contains the
CONFIG_CFG80211_EXTRA_REGDB_KEYDIR directory itself, and this check used
to filter it out. After the check was removed, the extra keydir option
no longer works, failing with the following message:

od: 'standard input': read error: Is a directory

This commit restores the check to make extra keydir work again.

Fixes: 715a1233

 ("wireless: don't write C files on failures")
Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>